A headless tadpole galaxy: the high gas-phase metallicity of the ultra-diffuse galaxy UGC 2162

TL;DR

This study confirms that the ultra-diffuse galaxy UGC 2162 has high gas-phase metallicity, challenging typical expectations and suggesting a link between UDGs and metal-rich galaxy phases.

Contribution

It provides the first spectroscopic confirmation of high metallicity in UGC 2162, highlighting its role as a potential link between UDGs and metal-rich galaxy states.

Findings

UGC 2162 has high metallicity (12 + log(O/H) ≈ 8.52).

UGC 2162 is an outlier in the mass-metallicity relation.

UGC 2162's properties suggest a connection to galaxies in different star-formation phases.

Abstract

The cosmological numerical simulations tell us that accretion of external metal-poor gas drives star-formation (SF) in galaxy disks. One the best pieces of observational evidence supporting this prediction is the existence of low metallicity star-forming regions in relatively high metallicity host galaxies. The SF is thought to be fed by metal-poor gas recently accreted. Since the gas accretion is stochastic, there should be galaxies with all the properties of a host but without the low metallicity starburst. These galaxies have not been identified yet. The exception may be UGC 2162, a nearby ultra-diffuse galaxy (UDG) which combines low surface brightness and relatively high metallicity. We confirm the high metallicity of UGC 2162 (12 + log(O/H) = 8.52+0.27-0.24 ) using spectra taken with the 10-m GTC telescope. GC2162 has the stellar mass, metallicity, and star-formation rate (SFR) surface density expected for a host galaxy in between outbursts. This fact suggests a physical connection between some UDGs and metal-poor galaxies, which may be the same type of object in a different phase of the SF cycle. UGC 2162 is a high-metallicity outlier of the mass-metallicity relation, a property shared by the few UDGs with known gas-phase metallicity.